The present invention relates in general to amplifier circuits and, more particularly, to setting a bias operating point for a single-ended amplifier during power-up.
Amplifier circuits are commonly used in electronic circuit design for receiving a single-ended input signal and providing an amplified single-ended output signal. The single-ended amplifier may be used for detecting low-level signals as low as few millivolts. The amplifier requires a power supply potential say V.sub.DD =5.0 volts to perform the amplification operation. The input of the amplifier circuit typically has a high input impedance and a low output impedance. The high input impedance produces a time delay for the input signal to propagate to the output especially during the time when the amplifier is powering up.
Single-ended amplifiers implemented in CMOS have an advantage of high speed operation and substantially no input offset voltage. Moreover, CMOS single-ended amplifiers have relatively simple designs requiring minimum number of transistors.
CMOS single-ended amplifiers have disadvantages such as sensitivity to noise and consumption of appreciable standby current. One way to reduce the power consumption in the CMOS single-ended amplifier is to select high impedance transistors and high value resistors. Another way of reducing power consumption is to use power cycling to shut-off the amplifier when not in use. The problem with power cycling the CMOS single-ended amplifier is the long settle-down delay due to the RC time constant at its input. A feedback resistor may be connected from the output of the amplifier back to its input for establishing a DC bias at both input and output terminals. The DC bias varies according to variations in thresholds and power supply potential. The self-bias level at the input of the amplifier does not reach an operational point until the amplifier powers up which may take several milliseconds. It is desirable to allow the input signal to propagate through the amplifier as rapidly as possible even during power-up.
Hence, a need exists to minimize the propagation delay during power-up and to isolate the supply noise from reaching the amplifier input.